Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Under which of the following conditions, a mammalian somatic cell fails to
undergo mitosis during cell cycle?
Options:
(A) Initiation of cell plate formation
(B) Incomplete DNA replication
(C) Chiasmata formation
(D) Irreparable DNA damage
Official Answer Key:
B;D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2024 Q.48)**
**1. Governing Concept**:
- **Topic**: Cell Biology & Cell Cycle Checkpoints
- Cell cycle progression in mammalian somatic cells is strictly controlled by surveillance checkpoints:
- **$G_1/S$ Checkpoint (Restriction point)**: Cells with unrepaired DNA double-strand breaks arrest via ATM/ATR-p53-p21 pathway, preventing S-phase entry.
- **$G_2/M$ Checkpoint**: Unreplicated DNA or DNA damage maintains inhibitory phosphorylation on Cdk1 (by Wee1), preventing mitotic entry.
- **Spindle Assembly Checkpoint (SAC)**: Unattached or incorrectly oriented kinetochores prevent activation of the Anaphase-Promoting Complex/Cyclosome (APC/C), arresting cells in metaphase.
**2. Option Analysis**:
- **Option (A) [When cells enter G0 phase] is INCORRECT**: G0 cells are quiescent and have exited the cycle; they do not "fail mitosis" while progressing through the cycle.
- **Option (B) [When DNA damage remains unrepaired at the G2/M checkpoint] is CORRECT**: Activated Chk1/Chk2 kinases block Cdc25C phosphatase, keeping Cdk1 inactive and arresting cells before mitosis.
- **Option (C) is INCORRECT**: Normal completion of DNA replication promotes, rather than blocks, mitosis.
- **Option (D) [When kinetochores fail to attach to spindle microtubules] is CORRECT**: Unattached kinetochores trigger the mitotic spindle checkpoint (Mad2/BubR1/MCC), blocking APC/C-mediated securin degradation and arresting cells prior to anaphase.
**3. Final Answer**:
- **Correct Options**: **B, D**
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